Noise margin information for power control and link adaptation in IEEE 802.11h WLAN

ABSTRACT

A method and apparatus for adjusting the transmission power level or transmission data rate between a plurality of stations located within the coverage area of a basic service set (BSS) or in an independent basic service set (IBSS) in a wireless local area network (WLAN). The receiving station extracts a transmission data rate from an incoming signal, determines a signal-to-noise ratio (SNR) for the incoming signal, and then calculates noise margin information based on a difference between the SNR of the incoming signal and a minimum SNR MIN  for the extracted data rate. The noise margin is then transmitted back to the original transmitting station and using the noise margin information, the transmit power level and/or the transmission rate of this station may be adjusted accordingly.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional ApplicationsSer. No. 60/320,128 filed Jun. 29, 2001, the teachings of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to communication systems. Moreparticularly, the present invention relates to a system and method forproviding information to the transmitting wireless stations in an IEEE802.11 wireless local area network (WLAN) regarding the receivingwireless station's receiver capabilities.

BACKGROUND OF THE INVENTION

In general, there are two variants of wireless local area networks(WLAN): infrastructure-based and ad hoc-type. In the former network,communication typically takes place only between the wireless nodes,called stations (STA_(i)), and the access point (AP), whereascommunication takes place between the wireless nodes in the latternetwork. The stations and the AP, which are within the same radiocoverage, are known as a basic service set (BSS).

The IEEE 802.11 standard specifies the medium access control (MAC) andphysical layer (PHY) characteristics for a wireless local area network(WLAN). The IEEE 802.11 standard is defined in International StandardISO/IEC 8802-11, “Information Technology—Telecommunications andinformation exchange area networks”, 1999 Edition, which is herebyincorporated by reference in its entirety.

Currently, the IEEE 802.11 does not provide a mechanism to enabledynamic transmit power control between wireless stations within a BSS.Typically, each 802.11 STA uses a fixed transmission power level for allthe frame transmissions throughout its lifetime. Accordingly, there is aneed for a transmit power level and transmission rate adjustmentmechanism that can be implemented within the proposed 802.11aPHY/802.11h MAC implementation without much complexity.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method of estimatingreceiver capability information by a receiving station, and conveyingthis information back to the transmitting station. The transmittingstation is thereby enabled to determine accurately the transmissionpower level or to adjust transmission rate in a wireless local areanetwork (WLAN) for future data transmissions to the receiving station.Receiver capability information, hereinafter known as “Noise Margin”, isthe receiving wireless station's receiver capabilities, including thelocal noise/interference level, the channel path loss and the channelconditions. Generally, the noise margin is the ratio of the receivedsignal power to that required for effective communication.

According to one illustrative embodiment of the present invention, amethod for determining the transmission power level and/or transmissionrate between a plurality of stations located within the coverage area ofa basic service set (BSS) in a wireless local area network (WLAN) isprovided. The method includes the steps of: extracting the data rate ofa transmission from an incoming frame; calculating the signal-to-noiseratio (SNR) of the received frame; calculating the noise margin for theextracted data rate based on the difference of the SNR and the minimumacceptable SNR (SNR_(MIN)); transmitting the noise margin information tothe transmitting station and adjusting the transmit power level or thetransmission rate for future data transmitted by the transmissionstation and/or other stations in the BSS according to the receivedcalculated noise margin.

The present invention also provides an apparatus with a powermeasurement circuit for determining the received power level between aplurality of stations located within the coverage area of a basicservice set (BSS) in a wireless local area network (WLAN). The apparatusincludes a receiver circuit for demodulating an incoming signal; an SNRmeasurement circuit for measuring the received SNR of the incomingsignal received therein; a processor, coupled to the power measurementcircuit, for calculating noise margin based on the difference of the SNRand the minimum acceptable SNR ratio for the extracted data rate; amemory, coupled to the processor, for storing the calculated noisemargin information for a predetermined time period for a subsequentretrieval; and, a transmitter circuit coupled to the processor

The foregoing and other features and advantages of the invention will beapparent from the following, more detailed description of preferredembodiments as illustrated in the accompanying drawings in whichreference characters refer to the same parts throughout the variousviews.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram illustrating the architecture of awireless communication system whereto embodiments of the presentinvention are to be applied;

FIG. 2 illustrates a simplified block diagram of an access point andeach station within a particular basic service set (BSS) according tothe embodiment of the present invention;

FIG. 3 illustrates the format of an 802.11 frame, including themodification of the SERVICE field, that can be used to transmitinformation between stations according to an embodiment of the presentinvention; and

FIG. 4 is a flow chart illustrating the operation steps of selectivelyadjusting the power level according to an embodiment of the presentinvention

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, for purposes of explanation rather thanlimitation, specific details are set forth such as the particulararchitecture, interfaces, techniques, etc., in order to provide athorough understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.

FIG. 1 illustrates a representative network whereto embodiments of thepresent invention are to be applied. As shown in FIG. 1, an access point(AP) 2 is coupled to a plurality of mobile stations (STA_(i)), which,through a wireless link, are communicating with each other and to the APvia a plurality of wireless channels. A key principle of the presentinvention is to provide a mechanism to enable a transmitting station togain knowledge regarding the receiving stations' localnoise/interference, channel path loss and its performance capability.This is accomplished by the receiving station estimating the noisemargin, for example, at the time and for the rate at which a frame isreceived, and conveying this information back to the transmittingstation). As noted above, the noise margin is the ratio of the receivedsignal power to that required for a particular level or threshold ofcommunication, (hereinafter known as effective or reliable communicationbetween the stations), the level is based on a number of factorseffecting communication, such as error rates, SNRs, as well as any otherconventional measure of transmission performance. The noise margin isuseful in, but not limited to, minimizing the transmitter power or othertypes of power control such as saving the battery power, avoidinginterference to other systems, adjusting radio coverage and adjustingtransmission rate, by transmitting frames at just the right power leveland the right transmission rate. IEEE 802.11 Physical Layers (PHYs)define a plurality of transmission rates based different modulations andchannel coding schemes so that the transmitter of a frame can choose oneof the multiple rates based on the wireless channel condition betweenthe receiver and itself at a particular time. Typically, the lower thetransmission rate, the more reliable the transmission. It should benoted that the network shown in FIG. 1 is small for purposes ofillustration. In practice most networks would include a much largernumber of mobile stations.

The AP and each STA within the WLAN of FIG. 1 may include a system withan architecture that is illustrated in the block diagram of FIG. 2.Referring to FIG. 2, both the AP and STA may include a receiver 12, ademodulator 14, a SNR measurement circuit 16, a memory 18, a controlprocessor 20, a timer 22, a modulator 24, and a transmitter 26. Theexemplary system 10 of FIG. 2 is for descriptive purposes only. Althoughthe description may refer to terms commonly used in describingparticular mobile stations, the description and concepts equally applyto other processing systems, including systems having architecturesdissimilar to that shown in FIG. 2.

In operation, the receiver 12 and the transmitter 26 are coupled to anantenna (not shown) to convert received signals and transmit desireddata into corresponding digital data via the demodulator 14 and themodulator 24, respectively. The SNR measurement circuit 16 operatesunder the control of the processor 20 to determine the SNR, or similarcalculation to estimate noise margin, of a received frame. Processor 20thereafter calculates the noise margin based on the difference of theSNR (in dB) and the minimum acceptable SNR (SNR_(MIN)) for a particulardata rate. The minimum acceptable SNR for a particular data rate isdetermined by any of a number of methods (e.g., an error rate thresholdmay be used or the receiver may be configured with modulation errorestimate parameters). For example, ‘noise margin’ calculated in an802.11 compliant STA for a received PPDU may be calculated as theadditional noise/interference in the PPDU in dB that could be toleratedby this STA without affecting its correct reception. As an example, if areceiver determines that a received PPDU had a signal-to-noise ratio(SNR) of 30 dB and it was transmitted at 24 Mbps rate, then if thereceiver could correctly receive the same PPDU at 24 Mbps withsignal-to-noise ratio of 24 dB (SNR_(MIN)), then it would report a noisemargin of 6 dB. The noise margin is stored in the memory 18 that iscoupled to the processor 20 for subsequent retrieval. The timer 22 isused to eliminate the outdated noise margin information, which is storedin the memory 18. In the embodiment, the noise margin is updated as itmay change due to the varying nature of the wireless channel as well asthe potential mobility of WLAN STAs.

FIG. 3 represents the format of PHY Protocol Data Unit (PPDU) frame thatis used to convey the noise margin information between the stations. Asshown in the lowest part of FIG. 3, the noise margin information(represented by noise margin Present and noise margin) is transmitted inthe SERVICE field of the 802.11a/h PPDU frame. The SERVICE field of the802.11a is slightly modified to include the one-bit noise margin Presentfield and four-bit noise margin field. The original SERVICE field 30format of the PLCP Header 28 of 802.11a is found in the middle part ofFIG. 3. The one-bit noise margin Present field 32 is defined as 0 or 1,where the noise margin field 34 may be optional in a system and presenceof the noise margin information is indicated by setting the ‘NoiseMargin Present’ bit. Otherwise, the ‘Noise margin Present’ bit is set tozero. The noise margin field 34 is defined from 1 to 16, where eachvalue is in decibels (dB), for example, −6 dB to +10 dB, and the fieldvalue would be based on calculations on immediate prior frametransmitted from the current destination STA. In addition, the RXVECTORand TXVECTOR fields, which are passed between MAC and are modified toadd noise margin information. In particular, one parameter, NOISEMARGIN, is extracted from or inserted into the SERVICE field 30 in thePPDU. The most significant bit of the currently reserved bits would beset if a valid value in present in the remaining seven bits. Else, itwould be reset with a ‘0’ value. As those skilled in the art willrecognize the noise margin information could be sent by other means, forexample, in the MAC payload, as well as other portions of the header.

The noise margin field 34 transmitted in a current PPDU would correspondwith the noise margin measurements on the immediately prior framereceived from the STA that is the current PPDU's destination STA. Thenoise margin information is transmitted back to the transmitting stationand now this station based on the received noise margin of the receiveradjusts the transmission power or the transmission rate. It is noted,however, that a receiver may transmit any noise margin value it may deemappropriate to indicate to the transmitter its performance capability,presence of local interference or desire for power adjustment. Afterobtaining the noise margin by receiving frame(s), the receiving STA candetermine both the PHY rates as well as the transmission powerintelligently for its future transmission to that STA. Thus, thetransmission power level and rate are determined solely up to thetransmitting STA's discretion.

Now, the principle of operation steps according to the present inventionof determining the noise margin to determine the transmission powerlevel/transmission rate is explained hereafter.

Referring to FIG. 4, the inventive process includes the following steps:in step 100, a station STA2 receives a frame from a transmitting stationSTA1. In step 110, the STA2 extracts the data rate from the receivedframe. In step 120, the STA2 calculates the noise margin, which is thedifference between the received SNR and the SNR_(MIN), The noise margininformation is transmitted to STA1 in step 130. The noise margin is thenused to determine the transmitter power required to obtain the desiredcarrier-to-noise ratio within the transmitting station and the receiverin BSS. In step 140, the STA1 adjusts the transmission power leveland/or the transmission rate based on the adjustment level that wasdetermined in step 120.

Although a limited number of STAs is shown in FIG. 1 for illustrativepurposes, it is to be understood that the WLAN can supportcommunications between a much larger number of STAs. Thus, the number ofSTAs in the figure should not impose limitations on the scope of theinvention. In such event, each STA keeps track of the noise marginbetween other STAs within the BSS and to the AP, then each transmittingstation may use the noise margin to adjust the transmit power level asit transmits a frame to another STA or to the AP. With non-802.11e WLAN,a STA needs to keep track of the noise margin with the AP only as theSTA must transmit frames to its AP. Here, each transmitting station maywant to keep track of the noise margin with a selected number of STAs toreduce the complexity. In addition, to prevent using outdated and stalenoise margin information, the present invention may adopt the noisemargin information lifetime. To this end, whenever STA 2 updates thenoise margin with STA 1 by receiving a frame from STA 1, STA 2 sets atimer for each updated noise margin information using the timer 22 ofFIG. 2. Hence, the STA 2 will compare the most recent updated time ofthe frame with the current time when it is to transmit a frame.

While the preferred embodiments of the present invention have beenillustrated and described, it will be understood by those skilled in theart that various changes and modifications may be made, and equivalentsmay be substituted for elements thereof without departing from the truescope of the present invention. In addition, many modifications may bemade to adapt to a particular situation and the teaching of the presentinvention without departing from the central scope. Therefore, it isintended that the present invention not be limited to the particularembodiment disclosed as the best mode contemplated for carrying out thepresent invention, but that the present invention include allembodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method for transmitting data among stations ina wireless local area network (WLAN), comprising: determining, at areceiving station, a measure of received signal power of a signalreceived from a transmitting station; calculating by the receivingstation a noise margin based on a difference between the measure ofreceived signal power and a power measure required for reliablecommunication at a predetermined data rate of the signal; andtransmitting the noise margin from the receiving station to thetransmitting station.
 2. The method of claim 1, further including:receiving the noise margin at the transmitting station, and adjusting atleast one of a transmit power level and a transmission rate of thetransmitting station based on the noise margin.
 3. The method of claim1, wherein: determining the measure of received signal power includesdetermining a signal-to-noise ratio (SNR) of the received framesignal;and calculating the noise margin includes determining a differencebetween the SNR and a minimum acceptable signal-to-noise ratio(SNR_(MIN)).
 4. The method of claim 1, further comprising storing saidnoise margin in a memory medium at the receiver.
 5. The method of claim1, wherein the transmitting station is located within a basic serviceset (BSS) or an independent basic service set (IBSS).
 6. The method ofclaim 1, wherein the receiving station is a mobile unit located within abasic service set (BSS) or an independent basic service set (IBSS).
 7. Amethod for transmitting data among a plurality of stations locatedwithin the coverage area of a basic service set (BSS) in a wirelesslocal area network (WLAN) comprising: transmitting a first frame from atransmitting station; receiving the first frame at the receivingstation; determining, at the receiving station, a transmission data rateof the first frame; calculating, by the receiving station, asignal-to-noise ratio (SNR) of the first frame; calculating, by thereceiving station, a noise margin based on a difference between the SNRand a minimum acceptable signal-to-noise ratio (SNR_(MIN)) for the datarate; transmitting the noise margin from the receiving station;receiving the noise margin at the transmitting station; calculating, bythe transmitting station, at least one of a new transmit power level andnew transmission data rate based on the noise margin; and, adjusting, atthe transmitting station, at least one of a transmit power level and thetransmission data rate to the at least one of a new transmit power leveland a new transmission data rate.
 8. The method of claim 7, furthercomprising: transmitting, by the receiving station, a second noisemargin to a second station and adjusting at least one of a transmissionpower level and a transmission rate of the second station based on thesecond noise margin.
 9. The method of claim 7, wherein the receivingstation or the transmitting station are mobile units.
 10. An apparatusfor determining the transmission power level between a plurality of nonaccess point stations located within the coverage area of a basicservice set (BSS) in a wireless local area network (WLAN), comprising:means for receiving an incoming signal from a first station; means fordemodulating said incoming signal; means for determining a transmissiondata rate and a measure of received signal power of the incoming signal;means for calculating a noise margin based on a difference between themeasure of received signal power and a power measure required for apredetermined threshold of communication for the transmission data rate;means for transmitting said noise margin to the said first station, and,means for storing said noise margin for a subsequent retrieval.
 11. Theapparatus of claim 10, further comprising means for adjusting at leastone of a transmit power level and a transmission rate of the apparatus,in response to said noise margin.
 12. The apparatus of claim 10, furthercomprising means for modulating signals indicative of said noise margin.13. The apparatus of claim 10, wherein the noise margin is stored for apredetermined time period.
 14. The apparatus of claim 10, wherein saidsignal is transmitted by a transmitting station located within a basicservice set (BSS) or an independent basic service set (IBSS).
 15. Themethod of claim 1, wherein the transmitting station is an access point(AP) located with a basic service set (BSS).
 16. The method of claim 1,wherein the receiving station is an access point (AP) located with abasic service set (BSS).
 17. The apparatus of claim 10, wherein thefirst station is an access point (AP) located with a basic service set(BSS).
 18. An apparatus comprising: a transmitter that is configured totransmit a signal at a transmit power level and a transmit data rate, areceiver that is configured to receive a noise margin corresponding toreceipt of the signal at a receiving station, wherein the noise marginis based on a difference between a measure of received signal power anda power measure required for reliable communication at a predetermineddata rate of the signal, and a processor that is configured to calculatean adjustment to at least one of the transmit power level and transmitdata rate based on the noise margin, and to correspondingly adjust theat least one transmit power level and transmit data rate.
 19. Theapparatus of claim 18, wherein the apparatus is a mobile station. 20.The apparatus of claim 18, wherein the apparatus is an access point (AP)located with a basic service set (BSS).
 21. A method for transmittingdata among stations in a wireless local area network (WLAN), the methodcomprising acts of: determining, at a receiving station, a transmissiondata rate and a measure of received signal power of a signal receivedfrom a transmitting station; calculating by the receiving station anoise margin based on a difference between the measure of receivedsignal power and a power measure required for a predetermined thresholdof communication for the transmission data rate; and transmitting thenoise margin from the receiving station to the transmitting station. 22.The method of claim 21, further comprising acts of: receiving the noisemargin at the transmitting station; and adjusting at least one of atransmit power level and a transmission rate of the transmitting stationbased on the noise margin.
 23. The method of claim 21, whereindetermining the measure of received signal power comprises acts of:determining a signal-to-noise ratio (SNR) of the received signal; andcalculating the noise margin includes determining a difference betweenthe SNR and a minimum acceptable signal-to-noise ratio.
 24. The methodof claim 21, further comprising an act of storing the noise margin in amemory at the receiver.
 25. The method of claim 21, further comprisingan act of storing the measure of received signal power in a memory atthe receiver.
 26. The method of claim 21, wherein the transmittingstation is located within a basic service set (BSS) which is one of aninfrastructural BSS and an independent basic service set (IBSS).
 27. Themethod of claim 21, wherein the receiving station is a mobile unitlocated within a basic service set (BSS) which is one of aninfrastructural BSS and an independent basic service set (IBSS).
 28. Areceiver for receiving data transmitted from a transmitting station in awireless local area network, the receiver comprising: a processorconfigured to: determine at the receiver, a transmission data rate and ameasure of received signal power of a signal received from thetransmitting station; calculate by the receiving station a noise marginbased on a difference between the measure of received signal power and apower measure required for a predetermined threshold of communicationfor the transmission data rate; and transmit the noise margin from thereceiving station to the transmitting station.
 29. The receiver of claim28, further comprising: a signal to noise measurement circuit configuredto determine the measure of received signal power by determining asignal-to-noise ratio (SNR) of the received signal; and the processor isconfigured to calculate the noise margin by determining a differencebetween the SNR and a minimum acceptable signal-to-noise ratio.
 30. Thereceiver of claim 28, further comprising a memory and the processor isconfigured to store the noise margin in the memory.
 31. The receiver ofclaim 28, further comprising a memory and the processor is configured tostore the measure of received signal power in the memory.
 32. Thereceiver of claim 28, wherein the receiver is a mobile unit locatedwithin a basic service set (BSS) which is one of an infrastructural BSSand an independent basic service set (IBSS)).
 33. An apparatus fordetermining the transmission power level between a plurality of wirelessstations in a wireless local area network (WLAN), the apparatuscomprising: a receiver configured to receive an incoming signal from afirst station; a demodulator configured to demodulate the incomingsignal; and a processor configured to determine a transmission data rateand a measure of received signal power of the incoming signal, calculatea noise margin based on a difference between the measure of receivedsignal power and a power measure required for a predetermined thresholdof communication for the transmission data rate, and transmit the noisemargin to the first station.
 34. The apparatus of claim 33, furthercomprising a memory wherein the processor is configured to store thenoise margin in the memory.
 35. The apparatus of claim 33, furthercomprising a memory wherein the processor is configured to store themeasure of received signal power in the memory.
 36. The apparatus ofclaim 33, wherein the processor is configured to adjust at least one ofa transmit power level and a transmission rate of the apparatus, inresponse to the noise margin.
 37. The apparatus of claim 33, furthercomprising a modulator configured to modulate signals indicative of thenoise margin.
 38. The apparatus of claim 33, wherein the noise margin isstored for a predetermined time period.
 39. The apparatus of claim 33,wherein the signal is transmitted by a transmitting station locatedwithin a basic service set (BSS) which is one of an infrastructural BSSand an independent basic service set (IBSS).
 40. The apparatus of claim33, wherein the first station is an access point (AP) located with abasic service set (BSS).
 41. A method for transmitting data amongstations in a wireless local area network (WLAN), the method comprisingacts of: determining, at a receiving station, a measure of receivedsignal power of a signal received from a transmitting station;calculating by the receiving station a noise margin based on adifference between the measure of received signal power and a minimumpower measure for a transmission data rate; and transmitting the noisemargin from the receiving station to the transmitting station.
 42. Themethod of claim 41, further comprising acts of: receiving the noisemargin at the transmitting station; and adjusting at least one of atransmit power level and a transmission rate of the transmitting stationbased on the noise margin.
 43. The method of claim 41, whereindetermining the measure of received signal power comprises acts of:determining a signal-to-noise ratio (SNR) of the received signal; andcalculating the noise margin includes determining a difference betweenthe SNR and a minimum acceptable signal-to-noise ratio.
 44. The methodof claim 41, further comprising an act of storing the noise margin in amemory at the receiver.
 45. The method of claim 41, further comprisingan act of storing the measure of received signal power in a memory atthe receiver.
 46. The method of claim 41, wherein the transmitting andreceiving stations are located within a basic service set (BSS) which isone of an infrastructural BSS and an independent basic service set(IBSS).
 47. An apparatus comprising: a transmitter that is configured totransmit a signal at a transmit power level and a transmit data rate toa receiving station; a receiver that is configured to receive a noisemargin corresponding to receipt of the signal at the receiving station,wherein the noise margin is based on a difference between a measure ofreceived signal power and a minimum power measure for a transmissiondata rate; and a processor that is configured to calculate an adjustmentto at least one of the transmit power level and transmit data rate basedon the noise margin, and to correspondingly adjust the at least onetransmit power level and transmit data rate.
 48. The apparatus of claim47, wherein the apparatus is a mobile station.
 49. The apparatus ofclaim 47, wherein the apparatus is an access point (AP) located with abasic service set (BSS).
 50. A station in a wireless local area network(WLAN), the station comprising: a processor configured to determine ameasure of received signal power of a signal received from atransmitting station; calculate a noise margin based on a differencebetween the measure of received signal power and a minimum power measurefor a transmission data rate; and transmit the noise margin to thetransmitting station.
 51. The station of claim 50, further comprising areceiver configured to receive the noise margin, wherein the processoris configured to adjust at least one of a transmit power level and atransmission rate based on the noise margin.
 52. The station of claim50, comprising a signal-to-noise measurement circuit configured todetermine a signal-to-noise ratio (SNR) of the received signal, whereinthe processor is configured to determine a difference between the SNRand a minimum acceptable signal-to-noise ratio.
 53. The station of claim50, further comprising a memory, wherein the processor is configured tostore the noise margin in the memory.
 54. The station of claim 50,further comprising a memory, wherein the processor is configured tostore the measure of received signal power in the memory.
 55. Thestation of claim 50, wherein the station is located within a basicservice set (BSS) which is one of an infrastructural BSS and anindependent basic service set (IBSS).